Patent GB-999905-A (GB)
Title: Apparatus responsive to the electrical conductivity of a liquid
IPC: G05D21/02
Filed: 1962-03-09
Abstract: 999, 905. Determining physical qualities of materials. M. INGRAM. March 9, 1962, [June 27, 1961], No. 9220/62. Heading G1N. An electric system for monitoring the salinity of a liquid comprises a conductivity cell 17 in thermal proximity to the liquid to be monitored, the cell including a pair of electrodes 29, for immersing in the liquid, and a temperature responsive resistor 24 in series therewith, the cell being connected to a source of current, a meter circuit for indicating the current through the cell, and a relay circuit for actuating an indicator when the salinity exceeds a predetermined value. Preferably several similar cells are provided, as shown, in different portions of the liquid. Thermistor 24 has resistors R4, R5 connected thereto so that its temperature response may be made similar to that of the cell. A large resistance R2 is connected across electrodes 29 so that the conductivity zero on the meter will lie slightly above the meter zero, to indicate that the power supply is connected. Meter M can be connected across each cell in turn through selector switch SW1. In parallel with the meter across each cell is a resistor chain R3, R3<SP>1</SP>, P1. The sensitivity of the meter may be increased by closing switch SW8, one half of which shorts out resistor R3<SP>1</SP> and the other half of which shorts out resistor 114 in the meter circuit. When a cell is not connected to the meter the resistance of the meter circuit is compensated by resistor R11. Connected across each potentiometer P1 is a relay circuit 31, 34 which operates an alarm circuit when a predetermined salinity is reached. Alarm circuitry. The first two cells 17, 17a have a common alarm bell 45 and indicator lamp 50. In this arrangement, when one cell sets off the alarm the other cell remains unmonitored, except for periodic checks by the meter. Bell 45 may be disconnected by switch SW6 while the salinity is being corrected. Cells 17b, 17c have a common alarm bell 46 but separate lamps 52, 54. With switch SW5b in the position shown, when relay 34b operates lamp 52 is connected in series with resistor R10. The voltage drop across R10 actuates relay 59 to operate bell 46. Switch SW5b is then turned to its alternative position, connecting lamp 51 in circuit, and leaving lamp 52 connected, but disconnecting the bell circuit. Lamp 52 continues to glow until the salinity is corrected, and 51 remains on until switch SW5b is returned to its first position. Meanwhile the bell circuit is available for actuation by excess salainity in cell 17c. Cell 17d is associated with a dumping circuit as well as indicator lamps 55, 56 and bell 46. When relay 34d operates, normally energized relay 71 is disconnected and contacts 91, 92 assume their full-line positions. Lamp 56 is thus illuminated and bell 46 sounds. Terminals 87, 88 are provided for connection to a distant alarm device. Dumping circuit. When relay 34d operates to de-energise relay 71 a dumping valve (not shown) connected to terminals 93, 94 is de-energised, and opens to remove the contaminated liquid. When liquid is dumped lamp 102, and thermal flasher 85, are connected to the supply. To prevent continued dumping during a fault switch SW7 is operated to disconnect lamp 102 and flasher 85, and connect lamp 58. In addition the circuit to the dumping valve through terminals 93, 94 is re-established to close the valve. Meter checking and calibrating. Operation of switch SW9 shunts resistor 118 across the meter to simulate the thermistor circuit resistance, and connects resistor 119, equivalent to the impedance of a mid-scale salinity between the electrodes, in series with the meter. A check point marking may be inscribed on the meter scale. Switch SW10 substitutes a variable dummy load 121,122 in the meter circuit. Resistor 121 may be varied to check the operating characteristics over the full meter scale. A particular cell is selected for checking by switch SW1 and the meter reading at which relay 34 operates can be determined. If a relay is replaced its potentiometer P1 is re-calibrated, with switch SW10 in the dotted position, by adjusting potentiometer P1 until the relay operates, then moving the scale on potentiometer P1 to align the predetermined scale marking with the pointer. The meter scale and potentiometer scale may be logerithmic.